Metal nanowires are great of technological importance due to their potential applications in miniaturized electrical, thermal, and mechanical system because their structure and properties can be quite different than bulk materials. The recent molecular dynamics works predicted the existence of strong but ductile behavior of metal nanowires achieved by deformation twinning. However it has been not experimentally proven by the tensile test of metallic nanowires. Unfortunately, previous experiment only observed high strength but brittle fracture in nanowires. Thus, the presence of ductile deformation in nanowires by structural transformation is in question and the deformation mechanism of nanowire proposed by atomic scale simulations is still controversial.We experimentally observed ultra-strong but ductile deformation behavior of single crystalline Au, Pd and PdAu nanowires resulting from deformation twinning process. In-situ tensile tests were performed using Nanomanipulator and AFM force measurement equipped in FEI DualBeam system. Microstructural change was also characterized using TEM at different deformation stages. The real time observation of twin nucleation and twin migration of metallic nanowires was accomplished with quantitative stress-strain measurement. The yield stress of a <110> rhombic Au nanowire reaches up to 1.54 GPa at 4% elastic strain and then twin nucleation occurs with a sudden load drop down to 200 MPa. Followed by twin migration, structural reorientation of the <110> rhombic nanowires into the <100> square nanowires results in ductile elongation at about 41% at the constant stress of 200 MPa. The ultra-strong but ductile deformation by twinning process was also observed for Pd and PdAu nanowires.